In recent years, a display component in an image display device such as a television device is shifting from a conventional cathode ray tube to a thin display panel such as a liquid crystal panel and a plasma display panel. With the thin display panel, a thickness of the image display device can be reduced. Because the liquid crystal panel used in the liquid crystal display device does not produce light by itself, it requires a backlight unit as a separate lighting unit. The backlight unit includes at least a light source and an optical member that is configured to exert optical effects on light rays emitted by the light source and direct the light rays to the liquid crystal panel. An example of such an optical member is disclosed in Patent Document 1. The optical member disclosed in Patent Document 1 is a composite sheet for a backlight unit including a first prism sheet, a second prism sheet, and a protective sheet. Prism ribs of the first prism sheet are bonded to a lower surface of the second prism sheet. Prism ribs of the second prism sheet are bonded to a lower surface of the protective sheet.